Trace elements as proxies for seasonality

Trace elements as proxies for seasonality in a modern speleothem from NE India

By Elli Rose Ronay

Thesis

Submitted to the Faculty of the Graduate School of Vanderbilt University

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE in

Earth and Environmental Sciences August 31, 2018

Nashville, Tennessee

Approved:

Jessica L. Oster, Ph.D.

Steven L. Goodbred, Ph.D.

ACKNOWLEDGEMENTS

ii

I am endlessly grateful for the support I have received over the past two years from the entire Vanderbilt EES community, my family and friends. I would especially like to thank my advisor Dr. Jessica Oster for her patient and generous mentorship. I have benefitted immensely from the countless opportunities for scientific growth she has provided and I feel excellently prepared to continue with my PhD. Thank you to the Oster lab group, namely Izzy Weisman and Cameron de Wet, for the encouragement, valuable feedback, and camaraderie. I would also like to thank Dr. Sebastian Breitenbach and Dr. Steve Goodbred for their meaningful advice and contributions to my thesis. Lastly, I want to thank all of the EES graduate students for their constant support and for reminding me when it’s Tuesday.

LIST OF TABLES

iii

Table Page

1. MAW-0201 U-Th dating table ...3

LIST OF FIGURES

iv

Figure Page

1. Elevation map of Meghalaya, India and Mawmluh Cave passages ...2

2. Smoothed trace elements, δ¹⁸O, PDO index, and rainfall record ...5

3. Cross wavelet transforms of δ¹⁸O, precipitation, and trace elements ...6

4. Moving average and z-scores of December precipitation ...7

5. I-STAL experimental results ...8

6. Calculating RMS deviations from I-STAL experiments...9

1 Introduction

The Indian Summer Monsoon (ISM) is a critical aspect of the climate system in South Asia, bringing 70-80% of annual rainfall between the months of June and September. The summer monsoon drives the Indian economy via its effect on agricultural success and has clear societal impacts from water scarcity to disastrous excess (Kumar et al., 2013; Mallya et al., 2015). The amount of monsoon rainfall is modulated by variations in the strength and flavor of El Niño events and decadal scale variability in the Pacific (Joseph et al., 2013; Krishnamurthy &

Krishnamurthy, 2014; Di Lorenzo et al., 2008; Sen Roy, 2011). Weak monsoons have

historically been associated with Central Pacific (CP) El Niño events (Kumar et al., 2006) and this weakening is enhanced during the warm phase of the Pacific Decadal Oscillation

(Krishnamurthy & Krishnamurthy, 2014).

Acute water shortages can occur during the Indian dry season following years of weak monsoon rainfall (Das et al., 2009). These shortages are further exacerbated when dry season rainfall is negligible. In Northeast (NE) India, winter rainfall typically comes from storms trending from the west called western disturbances (Sen Roy, 2006; Yadav et al., 2013).

Although it has been suggested that variations in the Pacific Decadal Oscillation (PDO) can modulate these western disturbances and winter rainfall in NE India (Nageswara Rao, 1999; Sen Roy, 2006), comparatively little is understood about winter rainfall and its relationship to

broader internal ocean-atmosphere variability. A clear understanding of the influences on dry season rainfall in NE India in conjunction with monsoon strength will have important

implications for water use planning, especially as global temperatures rise.

Records of δ¹⁸O variability in central and NE Indian speleothems document monsoon variability throughout the region on decadal to millennial timescales (Berkelhammer et al., 2010;

Denniston et al., 2000; Kathayat et al., 2017; Lechleitner et al., 2017). However, speleothem δ¹⁸O in this region primarily records changes in large scale atmospheric processes (Johnson, 2011; Maher & Thompson, 2012; Pausata et al., 2011), and does not provide information about variations in rainfall amount. Specifically, recent work in Mawmluh Cave in NE India has demonstrated that δ¹⁸O in rainwater, cave drip water, and speleothems reflect variations in moisture transport on seasonal, decadal, and centennial to millennial timescales (Breitenbach et al., 2010; 2015; Myers et al., 2015; Lechleitner et al., 2017). Thus, additional proxies are needed to investigate rainfall amount variability and how this relates to changes in moisture transport.

Further, it is not fully known how variations in the seasonality of rainfall amount might be recorded in speleothem carbonate from this region, nor what might drive changes in rainfall seasonality on decadal or longer timescales (Lechleitner et al., 2017). A clearer understanding of changes in dry season rainfall and how this might contribute to cave drip water could have important implications for the interpretations of many published monsoon-region speleothem records, as the majority have been interpreted to reflect variations in monsoon-season rainfall (Kathayat et al., 2017; Raza et al., 2017). Here we use a high temporal resolution record of trace element ratios in a modern speleothem from Mawmluh Cave to investigate the utility of trace elements as proxies for rainfall amount and seasonality in NE India and further examine the relationships between the El Nino Southern Oscillation (ENSO), decadal scale Pacific SST variability and year-round rainfall in this water-sensitive region.

The ratios of trace elements to calcium, such as Mg/Ca, Sr/Ca and U/Ca in speleothems can provide information about environmental conditions and processes occurring above a cave,

2

including changes in water supply driven by rainfall amount (Fairchild & Treble, 2009). The concentrations of these trace elements in the dripwater that feeds a speleothem are influenced by the precipitation of carbonate in the epikarst and on the cave ceiling (prior carbonate

precipitation, or PCP). The extent of PCP is primarily controlled by water residence times in the epikarst, a function of rainfall amount, and cave ventilation (Sinclair et al., 2012; Tremaine &

Froelich, 2013; Wong et al., 2011). Trace element ratios have been suggested as monsoon strength proxies (Johnson et al., 2006), but they may be strongly influenced by conditions during the winter dry season when drip rates are slow and caves are ventilated by low pCO2 air from the surface. Thus, trace element ratios in speleothems from monsoon regions may provide important records of past rainfall during the dry season.

Site and Sample Description

Mawmluh Cave is located on the southern margin of the Meghalaya Plateau (25°15’36”N 91°52’48”E), 13.7 km from Mawsynram and 2.3 km from Cherrapunji (Sohra), two villages that alternate for the title of the wettest place on earth. This region receives 70-80% of its annual rainfall during the Indian Summer Monsoon season between June and September (JJAS),

averaging 8688 mm (max 19519 mm, min 5093 mm) rainfall from 1963-2013 (Lechleitner et al., 2017; Yang et al., 2016). The Meghalaya Plateau is the first orographic feature that north

trending, moisture laden monsoon winds encounter as they move inland (Figure 1A), which induces rainfall on its southern edge (Lechleitner et al., 2017). Mawmluh Cave is located 1,320 m above sea level with 30-100 m of karstified limestone, dolostone, and sandstone overlying the cave, topped with 5-15 cm of soil (Breitenbach et al., 2015; Lechleitner et al., 2017).

Monitoring of Mawmluh Cave from 2007-2014 shows that seasonal, air temperature driven ventilation lowers cave air pCO2 in the winter dry season, while pCO2 increases in the cave through the summer monsoon season (Breitenbach et al., 2015). This pCO2 variability likely drives seasonal differences in speleothem growth rate due to differential CO2 degassing, such that faster speleothem growth rates occur during the dry season (Breitenbach et al., 2015;

James et al., 2015). Speleothem MAW-0201 was collected from the Hanging Gardens passage

A B

Figure 1. A. Elevation map of Meghalaya on inset map of India. Cherrapunji and Mawmluh cave lay on the southern margin of the Meghalaya Plateau. B. Map of Mawmluh Cave passages from Breitenbach et al., 2015. MAW-0201 was sampled from the Hanging Gardens passage.

3

of Mawmluh Cave in February 2013 and was actively growing at the time of collection (Figure 1B). MAW-0201 is 22 mm long at its growth axis and is composed of laminated aragonite, which precipitated on the base of a broken calcite stalagmite (Myers et al., 2015). Each ~380 μm thick lamination consists of a light-dark couplet resulting from seasonal growth rate induced density contrast. MAW-0201 has high uranium content (~40 ppm), which allowed dating at high precision, giving errors of 1 to 2.7 years for the record of growth from ~1964-2012 (Myers et al., 2015). Previous analysis of MAW-0201 revealed seasonal variations in speleothem δ¹⁸O. The lighter colored, thicker, microporous laminae attributed to dry season growth have generally higher δ¹⁸O than the darker, denser laminae of the wet season. These differences likely reflect seasonal variations in moisture transport (Breitenbach et al., 2010; Myers et al., 2015).

Methods

Six 15 mg samples averaging ~2 growth layer couplets were previously micromilled at 0mm (x2), 4mm, 15mm, and 20mm (x2) depth from the top of MAW-0201 for U-Th dating.

Each sample was dissolved fully in HNO3 and spiked with a calibrated solution of ²²⁹Th, ²³³U, and ²³⁶U. Cation exchange chromatography was then performed at the Berkeley Geochronology Center (BGC) to separate the U and Th from other components of the solution. These samples were analyzed at the BGC using a Thermo NEPTUNE Plus multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) and the age model was constructed using the COPRA algorithm (Breitenbach et al., 2012; Myers et al., 2015). U-Th dating methods are described in full in Myers et al., (2015), and all data are available in Table 1.

In this study, trace element concentrations were measured on a thin section of the MAW- 0201growth axis using laser ablation line scans with a Photon Machines Excimer laser coupled to a ThermoFisher iCAP Qc quadrupole ICP-MS at Vanderbilt University. Laser ablation was performed along the growth axis using a 20 µm x 100 µm rectangular spot at 20% laser power using a repetition rate of 15 Hz and a scan speed of 5 µm/s. The line scans followed a pre- ablation step that was conducted over the sample path at a speed of 10 µm/s at 50% laser power and a repetition rate of 15 Hz. The multi-element synthetic glass standard, NIST SRM 612, and the MACS3 synthetic pressed aragonite powder were analyzed at the beginning and end of the run, and NIST SRM 612 was used for elemental quantification. Data were processed using Iolite software package (Paton et al., 2011; Woodhead et al., 2007).

In order to compare the MAW-0201 trace element ratios with instrumental climate data, a Gaussian kernel with bandwidth 0.15 was used to smooth and down-sample the laser ablation

Table 1. U-series dating table for the MAW-0201

4

data to a monthly time step (Rehfeld et al., 2011). A bandwidth of 0.15 was chosen to smooth the densely sampled trace element records without oversmoothing and losing seasonal variability.

Prior to smoothing, Mg points higher than 2 standard deviations from the mean were removed (98 values above 80.3 ppm Mg). These high Mg measurements appeared as spikes in the Mg time-series and were likely derived from micron-scale soil particles embedded in the stalagmite (Jamieson et al., 2016; Belli et al., 2016). With the smoothed time series for each trace element, we performed continuous wavelet transforms (CWT) which illustrate the different periodicities that form the overall record in a frequency-time space using the wavelet coherence toolbox for MATLAB (Grinsted et al., 2004). We use the Morlet wavelet for all analyses and pad each time series with the mean value of the data to reduce the edge effects from the cone of influence.

We used the I-STAL speleothem trace element forward model to investigate mechanisms for producing the observed trace element variations in MAW-0201 (Stoll et al., 2012). I-STAL models the effects of water-rock interaction and PCP on cave dripwater and speleothem

chemistry. The model uses carbonate trace element partition coefficients and measured cave variables including drip interval, cave pCO2, temperature and initial CaCO3 saturation to build a record of trace element ratios in the resulting cave dripwater. We used I-STAL to forward model Mg/Ca in a pseudostalagmite during years characterized by strong versus weak monsoons and dry versus wet winters. We match drip rate monitoring data from Breitenbach et al. (2015) with contemporaneous rainfall data from nearby Cherrapunji (http://climexp.knmi.nl) to estimate an empirical relationship between rainfall and both monsoon and dry season drip intervals. We used this relationship to calculate drip interval, I (s), from the instrumental record of precipitation, P (mm), shown by

I = 337.25e^-0.002P.

The Mawmluh Cave monitoring study from 2007-2014 gives us pCO2, cave air

temperature, and drip rate to utilize in the model. However, data from the cave monitoring are limited during the monsoon season (JJAS) as flooding in some passages prevents access to the cave (Breitenbach et al., 2015). Thus, we use two measurements of cave air pCO2 taken in May 2010 and May 2012 (534 and 1049 ppm, respectively) to estimate summer cave air pCO2. The dry season cave pCO2 measurements are also irregularly sampled, so we estimate monthly pCO2

for the rest of the year from available data primarily from January and February, averaging 467 ppm CO2 (n=11). The temperature in Mawmluh cave over the 2007-2014 monitoring period ranged from 15°C to 22°C while the temperature at the surface varied between <1°C and 30°C.

Sensitivity experiments using observed seasonal temperature variability in Mawmluh Cave indicate that temperature does not have a significant influence on trace element variability in the I-STAL model relative to the influence of rainfall (drip interval) (making sensitivity figures for Supplementary Material). Thus, we hold temperature constant at 19°C which is the average of all temperature measurements in the Hanging Gardens passage taken from 2011-2014 (Breitenbach et al., 2015). Initial [Ca] is held constant at 111 ppm based off of dripwater ICP-MS

measurements (supplement), and the partition coefficient for Mg in aragonite (DMg) is taken from Wassenburg et al., (2016).

Results and Discussion

Trace elements are reported in ratios relative to Ca (mmol/mol). Trace element to Ca ratios Mg/Ca, U/Ca, Sr/Ca and Ba/Ca (Figure 2) show seasonal variability throughout the record and are all positively correlated (0.48 –0.85) except Sr/Ca and U/Ca. This correlation suggests

5

that U is primarily sourced from the bedrock rather than from soil organic matter and is

responsive to PCP. A soil U source would result in an anti-correlation between U/Ca and Mg/Ca and Sr/Ca as peaks in U/Ca would occur during the monsoon season, when organic material is more likely to wash into the cave, but when PCP, and thus Mg/Ca and Sr/Ca in drip water, are at a minimum (Johnson et al., 2006). The seasonal variability in these trace elements is, therefore, likely driven primarily by PCP (Tremaine & Froelich, 2013; Johnson et al., 2006). Between 1976 and 1999, the amplitude of this seasonal variability in Mg/Ca, U/Ca and Ba/Ca is smaller, and each dips to a local minimum at 1985. Sr/Ca shows both seasonal and subseasonal scale

Figure 2. Top to bottom: δ¹⁸O (purple) with LOESS smoothed line in bold), PDO index (black,

http://research.jisao.washington.edu/pdo/) with LOESS smoothed line in bold, gaussian kernel smoothed Ba/Ca (teal), Sr/Ca (green), U/Ca (blue) and Mg/Ca (orange), rainfall (mm). Blue bar highlights when PDO index is positive.

6

variability. Sr/Ca and Ba/Ca have overall decreasing trends throughout the record, which is not present in U/Ca, and only evident in Mg/Ca until ~1985. The record of annual rainfall amounts from 1964-2013 has no significant long-term trend (Figure 2), which implies that the overall decreasing trends in Sr/Ca and Ba/Ca, and for the early half of the record in Mg/Ca, are likely caused by something unrelated to rainfall amount. For example, exposing fresh dolomitized host- rock along a flow path due to seismic activity or land use change may explain why the Mg/Ca slope halts around 1985. Fresh dolomite, having relatively higher Mg than a weathered flow path or limestone, may allow more Mg to enter the dripwater after exposure. If the Sr/Ca and Ba/Ca are not replenished by the new exposure, as with Mg in dolomite, the ratios continue decreasing (Fairchild et al., 2006).

Continuous wavelet transforms of Mg/Ca and U/Ca show the strong, significant seasonal (1 year) signal consistently throughout the record (Figure 3). The strength of this seasonal signal diminishes between ~1976 and 1995, when the amplitude of seasonal variability also decreases (Figure 2) but remains significant throughout. Before 1976, ENSO scale (2-7 year) periodicities in these trace elements are significant and strong. After ~1981, 4-8 year periodicities return in U/Ca and 2-4 year periodicities become strong and significant in Mg/Ca starting after 1990.

Similar patterns are seen in the MAW-0201 δ¹⁸O CWT, where ENSO-scale periodicities are not significant during the period when PDO-index is positive (Myers et al., 2015). The Sr/Ca and Ba/Ca CWTs (Figure 3) also show significant annual periodicity, but with less strength and continuity than in Mg/Ca and U/Ca.

The variability in trace element ratio seasonal amplitudes and changes in the strength of seasonal periodicities that occur in concert with changes in the PDO are particularly striking. We see this seasonal signal weakening and amplitudes decreasing between ~1976 and 1995, which corresponds to the interval of time when the PDO index is positive, between 1977-1998 (Mantua

& Hare, 2002). Within this time period, there is an increase in average rainfall in Cherrapunji during December, a typically dry month (Figure 4), from 10 mm pre-1977 to 31 mm from 1977-

Figure 3. Cross wavelet transforms of trace elements, precipitation and δ¹⁸O in MAW-0201. Signals outlined in black are significant at the 95% confidence interval, brighter colors signify a stronger periodicity.

7

1998 and a decrease back to 7 mm post-1998. Cave air pCO2 is lower in the winter due to temperature driven seasonal ventilation (Breitenbach et al., 2015). This seasonal variability in pCO2 allows faster speleothem growth rates in the winter, despite lower rainfall and drip rates. In the following section, we test how these variables may affect seasonal amplitudes of trace

element ratios, particularly how seasonal changes in rainfall may be recorded in speleothem trace elements when there is a growth rate bias towards the dry season.

To test the relative effects of changing monsoon and winter rainfall on trace element ratios in Mawmluh Cave speleothems, we use the forward model, I-STAL (Stoll et al., 2012).

We model a strong monsoon with monthly drip intervals from 0.001-25s, calculated from Equation 1, which corresponds to 13400 mm of JJAS rainfall. This rainfall amount is 2 standard deviations greater than mean JJAS rainfall and ~1000 mm greater than any of the monsoons on record except for 19519 mm in 1974. To model a weak monsoon, we draw from the 2013

Figure 4. Top: Triangular moving average of December precipitation (mm) and Bottom: Z-score of December precipitation throughout the MAW-0201 record. Red points are above 2 standard deviations from mean December rainfall.

8

monsoon season, which had the lowest JJAS rainfall during the record at ~5100 mm. This rainfall amount corresponds to 20-32s drip intervals, calculated from Equation 1, during the monsoon months.

The “dry winter” months are characterized by no DJF rainfall. We model this with a drip interval of 337s, measured during the monitoring study, which increases to 345s and 350s in the second and third months of < 1mm rainfall. In the “wet winter” scenarios, we input a drip interval of 311s for each month, which correspond to 40 mm of rainfall based off of equation 1 and monitoring data (Breitenbach et al., 2015), totaling 120 mm DJF rainfall.

We use the strong monsoon and dry winter scenario as the control for the I-STAL forward modeling experiments. We first take the root mean square deviation from the annual mean for each season’s dripwater Mg/Ca in this control scenario. We then add these RMS deviations to get an average amplitude of Mg/Ca for the control. The same process is repeated for the weak monsoon and wet winter experiments, which we compare to the control Mg/Ca (Figure 6). All deviations from the control result in decreases in amplitude, meaning an induced

Figure 5. Mg/Ca from I-STAL forward modeling experiments of two years, using pCO2 and drip intervals from the Breitenbach et al. (2015) monitoring study. A: Strong versus weak monsoon, with dry winter. B:

Strong monsoon with "dry" versus "wet" winter.

A

B

9

weak monsoon will allow more PCP in the epikarst and increase the summer-associated Mg/Ca trough or a wet winter will decrease PCP and decrease the dry season Mg/Ca peak. The weak monsoon season (5100 mm) results in a 5.7% amplitude decrease (Figure 5A). The wet winter experiment (120 mm DJF) results in a 9.4% RMS change (Figure 5B). Together, the weak monsoon and wet winter scenarios result in a 15.1% decrease from the strong monsoon and dry winter control.

Comparing these I-STAL forward modeling experiments with measured MAW-0201 Mg/Ca suggests the modeled changes in amplitude may be lower limits. Using the same RMS deviation amplitude process, we compare three year-long Mg/Ca segments from the MAW-0201 record: 1998-1999 (strong monsoon/dry winter), 1971-1972 (weak monsoon/dry winter), and 1984-1985 (strong monsoon/wet winter). During the 1998 monsoon season, Cherrapunji received 11632 mm rainfall, and in DJF of 1998/1999 Cherrapunji received no measurable rainfall.

Cherrapunji received 6915 mm rainfall in JJAS of 1971 and 0.2 mm rainfall in DJF 1971/1972.

The measured amplitude of variation in Mg/Ca is 30% smaller during the weak monsoon/dry winter period compared with the strong monsoon/dry winter period. In 1984, Cherrapunji received 11372 mm JJAS rainfall and in the winter of 1984/1985, Cherrapunji received 148 mm rainfall. The Mg/Ca amplitude from this 1984-1985 strong monsoon/wet winter year is 80%

lower than the 1998-1999 strong monsoon/dry winter Mg/Ca amplitude. Thus, the change in trace element ratio amplitude from a wet winter is greater than that of a weak monsoon.

With both the modeled and measured Mg/Ca values, the difference in the annual amplitude of Mg/Ca variability is affected more by increased winter rainfall than decreased summer rainfall. In the forward-modeled stalagmite, an increase in winter rainfall by just 100 mm has a larger influence on Mg/Ca amplitude than a decrease in summer rainfall of 1000 mm.

This diminished sensitvity to changes in monsoon season rainfall is likely related to the sheer volume of monsoon rainfall in Meghalaya and resulting low drip interval in Mawmluh Cave, even during a weak monsoon season. Even in 2013, the driest monsoon season covered by our record, Cherrapunji received over 5000 mm of JJAS rainfall. This rainfall volume, coupled with higher cave pCO2 during the monsoon season may mitigate the influence of PCP in the epikarst

Figure 6. Trace element amplitude comparison using RMS deviations from the annual mean. Winter and summer amplitudes are summed to get what we are calling amplitude in the text.

10

and cave on drip water Mg/Ca and the extent to which this is captured in speleothem carbonate.

Conversely, a proportionally larger rainfall increase during the dry season would have a large effect on PCP that would be readily recorded in a speleothem during the low pCO2 winter season.

Our trace element results and interpretations are corroborated by the MAW-0201 δ¹⁸O record. Converse to the trace element signals, the seasonal amplitude of the MAW-0201 δ¹⁸O signal increases during the warm phase of PDO (Figure 2). Seasonal differences in MAW-0201 δ¹⁸O are linked to changes in transport pathways and moisture source. The increased seasonal amplitude and overall higher values of δ¹⁸O during positive PDO can also be explained by increased winter rainfall. Analysis of winter rainfall at Cherrapunji indicates higher δ¹⁸O values which result from shorter and more variable vapor transport pathways than during the monsoon season (Breitenbach et al., 2010).

Relative to monsoon season precipitation, the influence of ENSO and Pacific Decadal Variability on Indian winter precipitation is comparatively understudied. One analysis of monthly winter precipitation across India between 1925 and 1998 found only a modest negative relationship between PDO, ENSO, and precipitation across peninsular India due to a shift of the Hadley cell’s descending arm over central India (Sen Roy 2006). In this study, NE Indian precipitation showed the closest, and positive, relationship with SSTs in the Bay of Bengal (Sen Roy, 2006). This positive relationship between winter rainfall and warm Bay of Bengal SST anomalies is explained by the formation of a Rossby-type atmospheric wave with a low pressure trough over NE India. The low-pressure trough allows convection and increased rainfall in NE India. This analysis used a low spatial-resolution precipitation dataset based on an 18-cell 5° grid covering the Indian subcontinent (Hulme, 1992). Northeastern India, however, has dramatic variability in elevation and climate that may not be captured by a coarse grid (Prakash et al., 2015). Our analysis, in contrast to this low-resolution study and based on measurements from a nearby meteorological station, suggests a measurable increase in winter precipitation on the edge of the Shillong anticline during the positive phase of the PDO (Figure 2). Our results are

consistent with the observation that El Niños are associated with an intensified subtropical westerly jet, also due to the intensification of the descending arm of the Hadley cell over central India (Yadav et al., 2013), which would strengthen western disturbances that bring winter rainfall to northern India (Dimri, 2013; Yadav et al., 2013). The warm phase of PDO enhances the effects of El Niños in this region, which may further intensify western disturbances.

Strengthened western disturbances and higher winter rainfall would lead to lower Mg/Ca and higher ¹⁸O values in speleothem carbonate precipitated during the corresponding winter months.

The warm phase of PDO and CP El Niño events are correlated with reduced summer monsoon rains in central India (Krishna Kumar et al., 2006; Krisnamurthy and Krishnamurthy, 2014). However, rainfall in NE India, including the Cherrapunji rainfall record from 1963-2013, does not show a consistent relationship with PDO or ENSO state (Myers et al., 2015). For example, 1974 is by far the wettest monsoon season at Cherrapunji between 1963 and 2013, but is considered a drought year in the All India Summer Monsoon Rainfall record (Parthasarathy et al., 1994). Despite this, MAW-0201¹⁸O shows a clear relationship with the PDO, the North Pacific Gyre Oscillation, and CP El Niño events, that reflects the variations in large scale atmospheric circulation that influences moisture transport to NE India (Myers et al., 2015). Our new trace element records suggest that this relationship between Mawmluh Cave speleothem proxies and the PDO may more closely reflect winter conditions above the cave. Thus,

11

Mawmluh Cave may be a unique candidate for continuing further investigation of the

relationship between broader oceanic-atmospheric circulation processes and winter rainfall in this region.

What is not unique to Mawmluh Cave, however, is seasonal ventilation, which occurs in caves across the world (James et al., 2015). Although monsoon regions are primarily located in the tropics and subtropics where seasonal temperature fluctuations are smaller than those experienced in the mid to high latitudes, temperature driven ventilation can still drive seasonal changes in speleothem growth rate (Breitenbach et al., 2015; Casteel & Banner, 2015; Wong et al., 2011). Seasonal changes in cave ventilation and speleothem growth can also arise via other mechanisms such as changes prevailing winds (Noronha et al., 2017). These potential seasonal biases in speleothem growth must be taken into account when interpreting proxy records. Our record suggests that speleothems from monsoon regions can exhibit growth and proxy

information that is biased toward the dry season, even though even rainfall volumes can be an order of magnitude lower than wet season rainfall. This finding has important implications for speleothem proxies interpreted as reflecting changes in summer monsoon strength, especially from slower growing speleothems where carbonate cannot be sampled at seasonal resolution.

Conclusions

Our results suggest that the amplitudes of trace element ratios in sub-seasonally resolved speleothems may record the seasonality of annual rainfall. Dry season rainfall may be an

important driver of proxy behavior (Mg/Ca, U/Ca, δ¹⁸O) in seasonally ventilated caves where speleothem growth is biased towards the dry season. In these ventilated caves, speleothem trace element ratios may be more sensitive to variability in dry season rainfall than wet season rainfall.

Therefore, we advocate for caution when interpreting these trace elements as primarily summer monsoon intensity or annual rainfall indicators. In this hydrologically extreme region, a weak monsoon still brings 1000s of mm of rainfall. The PCP process that drives variability in speleothem trace elements may not be sensitive enough to variability at this magnitude of rainfall, especially when cave air pCO2 is high.

Pacific decadal variability may affect winter rainfall in NE India and corresponding speleothem proxy records. Increasing high resolution speleothem records in the past will allow us to further investigate this relationship. Understanding dry season rainfall variability in the past may allow better mediation of drought conditions before they occur in the rainiest place on earth.

12

BIBLIOGRAPHY

Breitenbach, S. F. M., Adkins, J. F., Meyer, H., Marwan, N., Kumar, K. K., & Haug, G. H.

(2010). Strong influence of water vapor source dynamics on stable isotopes in precipitation observed in Southern Meghalaya, NE India. Earth and Planetary Science Letters.

https://doi.org/10.1016/j.epsl.2010.01.038

Breitenbach, S. F. M., Lechleitner, F. A., Meyer, H., Diengdoh, G., Mattey, D., & Marwan, N.

(2015). Cave ventilation and rainfall signals in dripwater in a monsoonal setting – a monitoring study from NE India. Chemical Geology, 402, 111–124.

https://doi.org/10.1016/j.chemgeo.2015.03.011

Casteel, R. C., & Banner, J. L. (2015). Temperature-driven seasonal calcite growth and drip water trace element variations in a well-ventilated Texas cave: Implications for speleothem paleoclimate studies. Chemical Geology, 392, 43–58.

https://doi.org/10.1016/j.chemgeo.2014.11.002

Dimri, A. P. (2013). Relationship between ENSO phases with Northwest India winter precipitation. International Journal of Climatology, 33(8), 1917–1923.

https://doi.org/10.1002/joc.3559

Fairchild, I. J., Smith, C. L., Baker, A., Fuller, L., Spo, C., Mattey, D., & Mcdermott, F. (2006).

Modification and preservation of environmental signals in speleothems, 75, 105–153.

https://doi.org/10.1016/j.earscirev.2005.08.003

Grinsted, A., Moore, J. C., & Jevrejeva, S. (2004). Application of the cross wavelet transform and wavelet coherence to geophysical time series. Nonlinear Processes in Geophysics, 11(5/6), 561–566. https://doi.org/10.5194/npg-11-561-2004

Hulme, M. (1992). A 1951-80 global land precipitation climatology for the evaluation of general circulation models. Climate Dynamics, 7(2), 57–72. https://doi.org/10.1007/BF00209609 James, E. W., Banner, J. L., & Hardt, B. (2015). A global model for cave ventilation and

seasonal bias in speleothem paleoclimate records. Geochemistry, Geophysics, Geosystems, 16(4), 1044–1051. https://doi.org/10.1002/2014GC005658

Johnson, K. R. (2011). Palaeoclimate: Long-distance relationship. Nature Geoscience, 4(7), 426–

427. https://doi.org/10.1038/ngeo1190

Johnson, K. R., Hu, C., Belshaw, N. S., & Henderson, G. M. (2006). Seasonal trace-element and stable-isotope variations in a Chinese speleothem: The potential for high-resolution

paleomonsoon reconstruction. Earth and Planetary Science Letters, 244(1–2), 394–407.

https://doi.org/10.1016/j.epsl.2006.01.064

Kathayat, G., Cheng, H., Sinha, A., Yi, L., Li, X., Zhang, H., et al. (2017). The Indian monsoon variability and civilization changes in the Indian subcontinent. Science Advances, 3(12), 1–

9. https://doi.org/10.1126/sciadv.1701296

Krishnamurthy, L., & Krishnamurthy, V. (2014). Influence of PDO on South Asian summer monsoon and monsoon-ENSO relation. Climate Dynamics, 42(9–10), 2397–2410.

https://doi.org/10.1007/s00382-013-1856-z

13

Kumar, K. K., Rajagopalan, B., Hoerling, M., Bates, G., & Cane, M. (2006). Unraveling the Mystery of Indian Monsoon Failure During El Nino. Science, 314(5796), 115–119.

https://doi.org/10.1126/science.1131152

Lechleitner, F. A., Breitenbach, S. F. M., Cheng, H., Plessen, B., Rehfeld, K., Goswami, B., et al.

(2017). Climatic and in-cave influences on δ18O and δ13C in a stalagmite from northeastern India through the last deglaciation. Quaternary Research, (2017), 1–14.

https://doi.org/10.1017/qua.2017.72

Maher, B. A., & Thompson, R. (2012). Oxygen isotopes from chinese caves: Records not of monsoon rainfall but of circulation regime. Journal of Quaternary Science, 27(6), 615–624.

https://doi.org/10.1002/jqs.2553

Mantua, N. J., & Hare, S. R. (2002). The Pacific Decadal Oscillation. Journal of Oceanography.

https://doi.org/10.1023/A:1015820616384

Myers, C. G., Oster, J. L., Sharp, W. D., Bennartz, R., Kelley, N. P., Covey, A. K., &

Breitenbach, S. F. M. (2015). Northeast Indian stalagmite records Pacific decadal climate change: Implications for moisture transport and drought in India. Geophysical Research Letters, 42(10), 4124–4132. https://doi.org/10.1002/2015GL063826

Noronha, A.L., Hardt, B.F., Banner, J.L., Jenson, J.W., Partin, J.W., James, E.W., Lander, M.A.,

& Bautista, K.K. (2017) Trade winds drive pronounced seasonality in carbonate chemistry in a tropical Western Pacific island cavae - Implications for speleothem paleoclimatology.

Geochemistry, Geophysics, Geosystems, 18, 384-399. doi:10.1002/2016GC006644.

Parthasarathy, B., Munot, A. A., & Kothawale, D. R. (1994). All-India monthly and seasonal rainfall series: 1871-1993. Theoretical and Applied Climatology, 49(4), 217–224.

https://doi.org/10.1007/BF00867461

Paton, C., Hellstrom, J., Paul, B., Woodhead, J., & Hergt, J. (2011). Iolite: Freeware for the visualisation and processing of mass spectrometric data. Journal of Analytical Atomic Spectrometry, 26(12), 2508. https://doi.org/10.1039/c1ja10172b

Pausata, F. S. R., Battisti, D. S., Nisancioglu, K. H., & Bitz, C. M. (2011). Chinese stalagmite δ18O controlled by changes in the indian monsoon during a simulated heinrich event.

Nature Geoscience, 4(7), 474–480. https://doi.org/10.1038/ngeo1169

Prakash, S., Mitra, A. K., Momin, I. M., Rajagopal, E. N., Basu, S., Collins, M., et al. (2015).

Seasonal intercomparison of observational rainfall datasets over India during the southwest monsoon season. International Journal of Climatology, 35(9), 2326–2338.

https://doi.org/10.1002/joc.4129

Raza, W., Ahmad, S. M., Lone, M. A., Shen, C.-C., Sarma, D. S., & Kumar, A. (2017). Indian summer monsoon variability in southern India during the last deglaciation: Evidence from a high resolution stalagmite δ 18 O record. Palaeogeography, Palaeoclimatology,

Palaeoecology, 485, 476–485. https://doi.org/10.1016/j.palaeo.2017.07.003

Rehfeld, K., Marwan, N., Heitzig, J., & Kurths, J. (2011). Comparison of correlation analysis techniques for irregularly sampled time series. Nonlinear Processes in Geophysics, 18(3), 389–404. https://doi.org/10.5194/npg-18-389-2011

14

Sen Roy, S. (2006). The impacts of ENSO, PDO, and local SSTS on winter precipitation in India. Physical Geography, 27(5), 464–474. https://doi.org/10.2747/0272-3646.27.5.464 Stoll, H. M., Müller, W., & Prieto, M. (2012). I-STAL, a model for interpretation of Mg/Ca,

Sr/Ca and Ba/Ca variations in speleothems and its forward and inverse application on seasonal to millennial scales. Geochemistry, Geophysics, Geosystems, 13(1), 1–27.

https://doi.org/10.1029/2012GC004183

Tremaine, D. M., & Froelich, P. N. (2013). Speleothem trace element signatures: A hydrologic geochemical study of modern cave dripwaters and farmed calcite. Geochimica et

Cosmochimica Acta, 121, 522–545. https://doi.org/10.1016/j.gca.2013.07.026 Wong, C. I., Banner, J. L., & Musgrove, M. (2011). Seasonal dripwater Mg/Ca and Sr/Ca

variations driven by cave ventilation: Implications for and modeling of speleothem paleoclimate records. Geochimica et Cosmochimica Acta, 75(12), 3514–3529.

https://doi.org/10.1016/j.gca.2011.03.025

Woodhead, J. D., Hellstrom, J., Hergt, J. M., Greig, A., & Maas, R. (2007). Isotopic and elemental imaging of geological materials by laser ablation inductively coupled plasma- mass spectrometry. Geostandards and Geoanalytical Research, 31(4), 331–343.

https://doi.org/10.1111/j.1751-908X.2007.00104.x

Yadav, R. K., Ramu, D. A., & Dimri, A. P. (2013). On the relationship between ENSO patterns and winter precipitation over North and Central India. Global and Planetary Change, 107, 50–58. https://doi.org/10.1016/j.gloplacha.2013.04.006

Yang, H., Johnson, K. R., Griffiths, M. L., & Yoshimura, K. (2016). Interannual controls on oxygen isotope variability in Asian monsoon precipitation and implications for paleoclimate reconstructions. Journal of Geophysical Research: Atmospheres, 121(14), 8410–8428.

https://doi.org/10.1002/2015JD024683